As the title says... I have a transparent (glass or full transmission in the principled shader) plane, but I see the reflection of my area-light although the material is not reflective. The only way it disappears is when I set the IOR on the plane to 1, which defeats the purpose of my setup. Any ideas? Bug?
All (smooth) surfaces in the real world become reflective at low angles of incidence, Eevee and cycles (with Principled shader) reproduce this, but Blender Render didn't.
As RUben says using a refraction BSDF will refract the light as if it entered a material of the same density of glass, without simulating the reflections that you would get when shining a light through real glass.